Bear with me here goes.......
The US standard railway gauge is 4'8.5"
Why?
because thats the gauge in England and expat Englishmen helped build the railway in the US.
Why did the English build them like that?
Because the first railways were built by the same people that built the pre railway, tram ways and they used the same odd gauge.
Why did they use that gauge?
Because the people who built the tramways built wagons and used the same jigs and tools, which used 4'8.5" wheel spacing.
So why did wagons use that wheel spacing?
Because if they used wheel spacing of a different measure the wheels would break on the old rutted roads.
Who built the roads?
Imperial Rome built many of the old roads for their legions and those roads have been used ever since.
Why were the ruts 4'8.5" apart?
Because Roman chariots started the ruts and everyone else had to follow for fear of breaking their wheels. All Roman chariots were alike as they were used all over the empire.
So the US railway gauge is derived from the spec for old Roman chariots!
Now why were chariots that size and gauge?
Because they were designed so that the rear end of two war horses would just fit between the spars.
Now lets jump forward a few thousand years to the space shuttle.
The two big booster rockets are made by Thiokol. The engineers who designed them would like to have made them bigger. However they had to transport them by rail and through tunnels barely wider than the track. The track as we now know is just about as wide as two horses backsides.
So we can say that the size of the boosters on what is arguably man's most advanced transport system was determined by the width of a horses a*se!
And you thought that beig a horses A*se wasn't that important!